This invention relates to magnetic separators and, more particularly, is concerned with materials for packing the interior of a separating chamber which in use is placed in a high-intensity magnetic field and which forms part of a magnetic separator for separating magnetisable particles from a fluid in which they are suspended. The invention is also concerned with a process for separating magnetisable particles from a fluid in which they are suspended.
The force exerted on a spherical particle of magnetisable material in a magnetic field is given by the formula: EQU F=X.sub.m (.pi.D.sup.3 /6) H(.delta.H/.delta.x)
where X.sub.m is the volume magnetic susceptibility of the material, D is the diameter of the particle, H is the magnetic field intensity and .delta.H/.delta.x is the rate of change of the magnetic field intensity with distance.
Heretofore the separating chamber has been packed with ferromagnetic material, usually having a particulate or filamentary nature, to provide within the separating chamber a large number of points or collecting sites at which the local magnetic field intensity is high, interspersed with points of low local magnetic field intensity. This arrangement provides within the separating chamber a magnetic field which changes rapidly with distance and, as can be seen from the above expression, a high magnetic field gradient gives rise to a large force on a magnetisable particle. A very non-homogeneous magnetic field is especially desirable for separating magnetisable particles which have low magnetic susceptibilities and/or small diameters. Examples of suitable packings are:
(a) filamentary ferromagnetic materials, such as steel wool, or
(b) particulate ferromagnetic materials such as spheres, cylindrical pellets or cubes of ferromagnetic materials or more irregular particles such as those which are obtained when a block of ferromagnetic material is subjected to the action of a milling machine, or
(c) a foam of ferromagnetic material.
Heretofore, the fluid containing the magnetisable particles to be separated has been passed through the separating chamber containing the ferromagnetic packing material, and, at the same time, a magnetic field has been applied to the material, so that the packing material has been magnetised and the magnetisable particles attracted to the collecting sites within the packing material. The field has then been reduced to zero, preferably by alternating the field and progressively reducing its amplitude so as to take the value of the magnetisation of the packing material around a smaller and smaller hysteresis loop, until the residual magnetism within the packing material is effectively reduced to zero. A clean fluid has then preferably been flushed through the packing material to remove the magnetisable particles which have been collected. However, some of the magnetisable particles tend to form closed magnetic loops within the packing material when under the influence of the applied magnetic field, and these loops are not generally broken by the above-described degaussing process. Thus some of the particles may still be attracted to the packing material after the field has been reduced to zero, and may not therefore be removed by flushing out the separating chamber with a clean fluid.
For example, if a suspension of particulate solid material containing ferromagnetic particles, such as, for example, fine iron filings or particles of ferromagnetic iron compounds, such as magnetite, haematite or pyrrhotite, is passed through the separating chamber, it may be extremely difficult, if not impossible, to remove these ferromagnetic particles from the collecting sites within the packing material to which they are attracted, when the magnetic field is reduced to zero. Thus a proportion of the collecting sites, previously available for the collection of magnetisable particles, will remain occupied when a suspension of particulate solid material is next passed through the separator, and the efficiency of the magnetic separation process is reduced. If this retention of magnetic particles at the collecting sites is allowed to continue the packing may eventually become almost completely blocked.